From the beginning of powered aviation, aircraft have been under certain flying conditions troubled by accumulations of ice on component surfaces of aircraft such as wings and struts. If unchecked, such accumulations can eventually so laden the aircraft with additional weight and so alter the airfoil configuration of the wings and control surfaces of that aircraft so as to precipitate an unflyable condition. Efforts to prevent and/or remove such accumulations of ice under flying conditions has resulted in three generally universal approaches to removal of accumulated ice, a process known generally as de-icing.
In one form of de-icing known as thermal de-icing, leading edges, are heated to loosen adhesive forces between accumulating ice and the aircraft component. "Leading edges" as used herein means those edges of an aircraft component on which ice accretes and are impinged upon by air flowing over the aircraft and having a point or line at which this airflow stagnates. Once loosened, this ice is generally blown from the aircraft component by the airstream passing over the aircraft. There are two popular methods of heating leading edges. In one approach known as electrothermal de-icing, an electrical heating element is placed in the leading edge zone of the aircraft component, either by inclusion in a elastomeric boot applied over the leading edge, or by incorporation into the skin structure of the aircraft component. This heating element is typically powered by electrical energy derived from a generating source driven by one or more of the aircraft engines and is switched on and off to provide heat sufficient to loosen accumulating ice. In small aircraft, a sufficient quantity of electrical power may be unavailable for use of electrothermal de-icing.
In the other heating approach, gases at elevated temperature from one or more compression stages of a turbine engine are circulated through the leading edges of components such as wings and struts in order to affect a de-icing or anti-icing effect. This approach is employed typically only in aircraft powered by turbine engines by draining off compressed air having an elevated temperature from one or more compressor stations of a turbine engine. This approach can result in reduced fuel economy and lower turbine power output.
The second commonly employed method for de-icing involves chemicals. In limited situations, a chemical has been applied to all or part of an aircraft to depress adhesion forces associated with ice accumulation upon the aircraft or to depress the freezing point of water collecting upon surfaces of the aircraft.
The remaining commonly employed method for de-icing is typically termed mechanical de-icing. In the principal commercial mechanical de-icing means, pneumatic de-icing, the leading edge zone or wing or strut component of an aircraft is covered with a plurality of expandable, generally tube-like structures, inflatable employing a pressurized fluid, typically air. Upon inflation, the tubular structures tend to expand substantially the leading edge profile of the wing or strut and crack ice accumulating thereon for dispersal into the airstream passing over the aircraft component. Typically, these tube-like structures have been configured to extend substantially parallel to the leading edge of the aircraft. These conventional low pressure pneumatic de-icers are formed from compounds having rubbery or substantially elastic properties. Typically, the material forming the inflatable tubes on such de-icer structures can expand or stretch by 40% or more during an inflatable cycle, thereby causing a substantial change in the profile the de-icer as well as in the leading edge to thereby crack ice accumulating on the leading edge. These conventional pneumatic de-icers require a large volume of air to inflate their highly expandable tubes and the time for inflating such tubes typically and historically has averaged from about two and six seconds. The distortion of the airfoil profile caused by inflation of the tubes can substantially alter the airflow pattern over the airfoil and adversely affect the lift characteristics of the airfoil. The rubber or rubber-like materials forming these conventional pneumatic de-icers typically are possessed of a Young's modulus (modulus of elasticity) of approximately 6900 Kpa. The modulus of elasticity of ice is variously reported as being between about 275,000 Kpa and about 3,450,000 Kpa. Ice is known to be possessed of an elastic modulus enabling typical ice accumulations to adjust to minor changes in contours of surfaces supporting such ice accumulations. While the modulus of elasticity of rubber compounds used in conventional de-icers is much lower than the modulus of elasticity typically associated with ice accumulations. The large expansion of conventional pneumatic de-icers has functioned to crack or rupture the structure of the ice accumulations thereby allowing such accumulations to be swept away by impinging windstreams.
Other mechanical means for effecting ice de-icing include electromechanical hammering such as that described in U.S. Pat. No. 3,549,964 to Levin et al. Concern respecting the susceptibility of the airfoil skin to stress fatigue upon being hammered over extended periods of time have functioned in part to preclude substantial commercial development or adoption of such technique.
Another electromechanical ice removal system is described in U.S. Pat. No. 4,690,353 to Haslim et al. One or more overlapped flexible ribbon conductors, each of which is folded back on itself, is embedded in an elastomeric material. When a large current pulse is fed to the conductor, the anti-parallel currents in the opposed segments of adjacent layers of the conductor result in interacting magnetic fields producing an electrorepulsive force between the overlapping conductor segments causing them to be separated near instantaneously. This distention tends to remove any solid body on the surface of the elastomeric material.
U.S. Pat. Nos. 4,706,911 to Briscoe et al and 4,747,575 to Putt et al disclose apparatus for de-icing leading edges in which an impulse of fluid under pressure is utilized to rapidly inflate an inflation tube positioned between a support surface and a sheet-like skin possessed of a substantially elevated modulus. The impulse of fluid is delivered to the inflation tube causing the high modulus skin to dislocate and then stop suddenly. Momentum imparted to the ice accumulations thereby causes additional ice movement which assists in ice detachment and dislodgement. The inflatable tubular structure in certain preferred embodiments is inflated within not more than about 0.1 second and preferably not more than about 0.5 milliseconds. FIG. 4 and the attendant description of U.S. Pat. No. 4,706,911 describe an ejector/pilot operated discharge valve suitable for use in such pneumatic impulse de-icers. In FIG. 7 and the attendant description of U.S. Pat. No. 4,747,575 there is described a chattering valve for use in a pneumatic impulse de-icer which delivers a rapid series of fluid pressure pulses to the inflatable tube of a de-icer apparatus affixed to a leading edge. Efforts to improve such pneumatic impulse de-icing systems have led to continuing efforts to improve valves for delivery of the desired fluid impulse.